Biceps brachii regional shear modulus following elbow flexion exercises at different muscle lengths

The biceps brachii (BB) muscle, due to its biarticular nature, undergoes substantial length changes during exercise. Muscle elongation, particularly when the shoulder is extended, increases passive tension and may impair force production due to passive insufficiency. The distal BB appears vulnerable to mechanical strain and injury during eccentric contractions. This study investigated regional changes in BB shear modulus (μ) through shear wave elastography before and after elbow flexion exercises performed in three shoulder positions: flexion (45°), neutral (0°), and extension (-45°). Eighteen resistance-trained males performed 3 sets of 10 repetitions at 80 % of their one-repetition maximum in each condition. The μ was measured in the proximal and distal regions of the long and short heads of the BB, pre and post-exercise. A three-way repeated-measures MANOVA revealed significant effects of joint position on muscle length (p < 0.05) and pre-exercise μ, with values increasing from flexion to extension. K-means clustering revealed heterogeneous individual responses, with emphasis on the distal region of the long head of the BB, showing μ reductions in 78.6 % of subjects. Post-exercise reductions in μ in well-trained men suggest region-specific mechanical responses potentially linked to muscle damage, particularly in the distal long head of the BB during shoulder extension. Shear wave elastography proved effective in detecting changes in BB stiffness, supporting its application in training and rehabilitation studies. Heavy exercises at long muscle lengths should be prescribed with caution in untrained individuals due to elevated mechanical stress.